We are working on a singular type of neurotoxin, that of Brazilian rattlesnake venom, protoxin, which requires two proteins that tend to form a complex, to express its high activity of blocking neuromuscular impulse transmission. We have found that upon separate or dissociated administration of the two proteins toxicity is still retained. We also found that one component of a similarly dual and synergistic mushroom cardiotoxin, volvatoxin A2, complexes with one of the crotoxin components (B, the strongly basic one) and greatly increases its toxicity. The toxicity of crotoxin shows differences in its nature from the two classes of neurotoxic action now recognized, alpha characterized by postsynaptic, and beta by presynaptic action, and exemplified by alpha and beta bungarotoxin. We intend to establish the actual mechanism of action of each of the components of crotoxin by use of radio-labeled derivatives (which have already yielded preliminary results); by further substitution of the components of the system by components of other (bacterial) toxins, and by chemically modified components; by further ultrastructural studies of target organs; by studying the apparent membrane-lipolytic action of crotoxin B; any by further protein structural investigations. We believe that to learn more about the mode of action of this quite singular neurotoxin may teach us more about neuromuscular impulse transmission.